Help me understand centertapping!

[Mattlee0037]

So I'm fairly new to all this. No real world building a tube amp experience yet, but in my reading and research I've seen this term come up a lot. The references so far have been filament wiring by running twisted wires from socket to socket using a grounded center tap or a pair of 100 Ohm resistors, and also that the 6.3v filament voltage can power the 12ax7's heater because of a centertap at pin 9? Anyone care to lay it out simply for a beginner what it is and what it does? Along with maybe an explanation of the false center tap resistor scheme?

[tribi9]

A center tap is your connection to ground for the heater wires. It can either be a wire on your PT or you can do an artificial tap if your pt doesnt have a centre tapped heater supply. Usually 2x100R 1/2 watt resistors.

If its a wire you can also elevate by adding DC voltage to it. On a cathode biased amp you simply connect your heater CT to the cathode of one your power tubes.

[EtherealWidow]

Center taps are useful for a couple of different things in tube amps.

Like tribi9 said, on a filament winding, it's useful for elevating the heater voltage, so instead of going from -3.15 to +3.15, you can have it go from 16.85 to 23.15. This places the heaters at a higher potential than the cathodes so there is no heater noise being inducted into the cathode.

In addition to what he said, there are also center taps on push-pull output transformers. The center taps go to the B+ and each of the ends goes to a plate of an output tube. The tube current flows through each section of the winding in opposing directions and through the center tap. When a signal is applied, each half of the winding is out of phase with each other. One half of the winding is conducting more, while the other is conducting less. This is then inducted to the secondary which pushes and pulls the speaker.

There are also center taps on the HV secondary winding oftentimes. In this instance, you ground the center tap. Then usually, you set up a full wave rectifier so that only the positive peaks are allowed to pass. Then you filter that, and you have your B+.

[martin manning]

The 12AX7 has two separate 6V filaments, one for each triode. One end of both filaments is connected to pin 9 and the other ends are connected to pins 4 and 5. When you wire them for operation on 6V, you join pins 4 and 5 together which connects them in parallel. For 12V operation, 12V is applied to pins 4 and 5 so two 6V filaments are wired in series.

[Mattlee0037]

Thanks for clearing this up! Very interesting about elevating the heater voltages!

[xtian]

Even if you create an artificial CT using 100R resistors, you can still elevate this via cathode DC, right?


One important thing, don't ask me how I know, is that you should not connect BOTH real and artificial center taps.

[pdf64]

you should not connect BOTH real and artificial center taps

What's the problem?
Surely you just get 3.15V across each resistor?

Center taps are useful ... for elevating the heater voltage, so instead of going from -3.15 to +3.15, you can have it go from 16.85 to 23.15. This places the heaters at a higher potential than the cathodes so there is no heater noise being inducted into the cathode

What is the benefit of using a real or artificial center tap for the Vdc elevation reference point, rather than referencing to one or other of the heater lines?
It seems to me that the key point is for the lowest heater voltage should be above the highest cathode voltage, see http://www.valvewizard.co.uk/heater.html
Pete